Alkanethiols on Platinum: Multicomponent Self-Assembled Monolayers
MARYLAND UNIV COLLEGE PARK DEPT OF PHYSICS
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We have studied the formation of self-assembled monolayers SAMs of n-alkanethiols on platinum thin films using X-ray photoelectron spectroscopy XPS, reflection-absorption infrared spectroscopy RAIRS, spectroscopic ellipsometry SE, and contact angle CA measurements. Specifically, SAMs of 1-hexanethiol, 1-dodecanethiol, and 1-octadecanethiol were grown on polycrystalline Pt films, and the effects of Pt surface preparation, deposition conditions, and solvent treatments on the initial quality and stability of the monolayer in air were investigated. The SAMs prepared under ambient conditions on piranha-cleaned and UVozone-cleaned substrates were compared to monolayers formed on template-stripped Pt in an inert atmosphere. We found that alkanethiols deposited from 1 mM ethanolic solutions on piranha-cleaned Pt formed densely packed monolayers in which alkyl chains were oriented close to the surface normal. Stored in the laboratory ambient, these monolayers were unchanged over about 1 week but were largely oxidized in about 1 month. No evidence was found of molecules being weakly bound within the monolayer or having undergone C-S bond scission however, three distinct sulfur states were observed for all samples in the XPS of the S 2p region. The lowest- and highest-binding-energy components are assigned to alkylthiolate and partially oxidized alkylthiolate species, respectively. The remaining S 2p component approximately one-third of the sulfur layer, intermediate in binding energy between the other two components, is attributed to a chemisorbed species with a S binding configuration distinct from the majority alkylthiolate for example, S bound to Pt bound to O, S with a different Pt coordination number, or S in an adsorbed disulfide.
- Organic Chemistry
- Atomic and Molecular Physics and Spectroscopy